1. Field of the Invention
This invention relates to meltblown fibers, meltblown fiber webs, and composite nonwoven fabrics that include meltblown fibers. The meltblown webs of the invention can be incorporated in composite fabrics suited for use in apparel, wipes, hygiene products, and medical wraps.
2. Description of Related Art
In a meltblowing process, a nonwoven web is formed by extruding molten polymer through a die and then attenuating and breaking the resulting filaments with a hot, high-velocity gas stream. This process generates short, very fine fibers that can be collected on a moving belt where they bond with each other during cooling. Meltblown webs can be made that exhibit very good barrier properties.
Meltblown fibers are most typically spun from polypropylene. Other polymers that have been spun as meltblown fibers include polyethylene, polyamides, polyesters, and polyurethanes. Polyester polymers, such as poly(ethylene terephthalate) (xe2x80x9cPETxe2x80x9d) and poly(trimethylene terephthalate) (xe2x80x9cPTTxe2x80x9d), are not well adapted for making fine meltblown fibers. In addition, due to polyester""s low degree of crystallization when formed in meltblown webs and due to polyester""s low crystallization temperature, thermally bonded meltblown polyester webs tend to be brittle and they exhibit relatively poor fluid barrier properties, especially when subjected to mechanical stress. U.S. Pat. No. 5,364,694 discloses the meltblowing of a blend of PET with another thermoplastic polymer, such as polyethylene, which is incompatible with PET and has a high crystallization rate and a low melt viscosity. The second polymer produces a xe2x80x9cviscosity-reducing effectxe2x80x9d that decreases the melt viscosity of the entire blend, so as to facilitate attenuation of PET when meltblown. U.S. Pat. No. 4,795,668 discloses the meltblowing of bicomponent fibers wherein one component is PET and the other component is a more thermally stable polymer such as polypropylene or polystyrene.
Meltblown fibers have been incorporated into a variety of nonwoven fabrics including composite laminates such as spunbond-meltblown-spunbond (xe2x80x9cSMSxe2x80x9d) composite sheets. In SMS composites, the exterior layers are spunbond fiber layers that contribute strength to the overall composite, while the core layer is a meltblown fiber layer that provides barrier properties. Traditionally, the spunbond and meltblown layers of SMS composites have been made of polypropylene fibers. For certain end use applications, such as medical gowns, it is desirable that SMS composite sheets have good strength and barrier properties, while also being as soft and drapeable as possible. While polypropylene-based SMS fabrics offer good strength and barrier properties, they tend not to be as soft and drapeable as is desirable for apparel products. Polypropylene-based SMS fabrics also have the limitation that they cannot be sterilized with gamma radiation because such fabrics are discolored and weakened when sterilized with gamma radiation, and because gamma radiation sterilization of polypropylene-based SMS fabrics generates unpleasant odors. A polymer fiber or fabric is generally considered to be not radiation sterilizable when sterilization of the fabric with gamma radiation causes a significant reduction in the strength of the fiber or fabric, noticeably changes the appearance of the fiber or fabric, or generates an objectionable odor. This inability to undergo gamma radiation sterilization presents a significant problem for polypropylene-based SMS fabrics because radiation sterilization is commonly used throughout the medical industry.
There is a need for finer polyester meltblown fibers that when formed into webs exhibit good barrier properties. There is a further need for meltblown polyester webs that are pliable and do not experience a significant loss in barrier properties when mechanically stressed.
The present invention is directed to a meltblown fiber and a web of meltblown fibers. The meltblown fiber of the invention comprises at least 20% by weight polyester selected from the group consisting of poly(ethylene terephthalate) having an intrinsic viscosity of less than 0.55 dl/g, and poly(trimethylene terephthalate) having an intrinsic viscosity of less than 0.80 dl/g. The meltblown fiber of the invention has an average effective diameter of less than 10 microns. Preferably, the intrinsic viscosity of the poly(ethylene terephthalate) is in the range of 0.20 to 0.50 dl/g and the intrinsic viscosity of the poly(trimethylene terephthalate) is in the range of 0.45 to 0.75 dl/g. More preferably, the intrinsic viscosity of the poly(ethylene terephthalate) is in the range of 0.25 to 0.45 dl/g and the intrinsic viscosity of the poly(trimethylene terephthalate) is in the range of 0.50 to 0.70 dl/g. Meltblown fibers of the invention are preferably formed into a meltblown web.
According to one preferred embodiment of the invention, the meltblown fiber is a multiple component fiber comprised of between 20% and 98% by weight of poly(ethylene terephthalate) and between 80% and 2% by weight of a second polymer component comprised of at least 10% of polyethylene polymer. Meltblown fibers of the invention are preferably formed into a multiple component meltblown web comprised of between 20% and 98% by weight of poly(ethylene terephthalate) and between 80% and 2% by weight of a second polymer component comprised at least 10% by weight of polyethylene polymer.
The present invention is also directed to a composite sheet having a first fibrous layer having a first side and an opposite second side, and a second fibrous layer bonded to the first side of the first fibrous layer. The first fibrous layer is a meltblown web comprised of at least 20% by weight polyester selected from the group consisting of poly(ethylene terephthalate) having an intrinsic viscosity of less than 0.55 dl/g, and poly(trimethylene terephthalate) having an intrinsic viscosity of less than 0.80 dl/g. The second fibrous layer is preferably comprised of at least 95% by weight of meltspun fibers. In the preferred embodiment of the invention, the composite sheet has a basis weight of less than 120 g/m2, and a hydrostatic head of at least 10 cm. According to a more preferred embodiment of the invention, at least 10% of the meltblown fibers in the first fibrous layer are multiple component fibers. More preferably the multiple component meltblown fibers have a low intrinsic viscosity polyester component and a polyethylene component. According to the invention, the meltspun fibers of the second fibrous layer can be multiple component fibers having a polyester component and a polyethylene component. The invention is also directed to garments made of the composite sheet of the invention.
The present invention is also directed to a meltblown fiber comprising at least 20% by weight polyester having a weight average molecular weight of less than 25,000. Preferably, the polyester has a weight average molecular weight in the range of 5,000 to 22,000. More preferably, the polyester has a weight average molecular weight in the range of 10,000 to 19,000.